1. Field of the Invention
The present invention relates to a steel plate reinforcing sheet and, more particularly, to a steel plate reinforcing sheet used to be adhesively bonded to steel plates used in a variety of industrial machines such as transportation machines.
2. Description of the Prior Art
In general, a steel plate for a shell of an automotive vehicle is manufactured in the form of a thin sheet having a thickness of generally in the range of 0.6 mm to 0.8 mm, for reduction in weight of a vehicle body. As is known, a steel plate reinforcing sheet comprising a restricting layer and a resin layer is adhesively bonded to the inside of the steel plate, to cure the resin layer, for the purpose of reinforcing the steel plate.
Generally, in such a steel plate, the steel plate reinforcing sheet is subjected to electrodeposition coating after the steel plate reinforcing sheet is adhesively bonded to it, so that the resin layer is cured by heating at the time of drying the coating.
This steel plate reinforcing sheet is known that a sheet-like resin layer formed of a thermally curable resin composition is laminated on a restricting layer formed of glass fiber cloth, thin metal sheet, and the like. As the thermally curable resin composition, epoxy resin composition (Cf. JP Laid-open (Unexamined) Patent Publication No. Hei 8-325349), and rubber-epoxy composition containing styrene-butadiene rubber, epoxy resin, and curing agent thereof (Cf. JP Laid-open (Unexamined) Patent Publication No. 11-246827) are proposed.
When these steel plate reinforcing sheets have a resin layer formed of epoxy resin composition, the reinforcing effect can be sufficient. However, the epoxy resin is a semisolid resin with low molecular weight, which is therefore highly temperature-sensitive. This property has disadvantages that use of the epoxy resin in a resin layer makes it difficult to peel off the resin layer from exfoliate paper, and also leads to low adhesion of the resin layer to the steel plate. In addition, there are disadvantages related to workability such as overflow of resin during storage and resin dripping during heat-curing.
On the other hand, in the rubber-epoxy composition, it is difficult to stably mix styrene-butadiene rubber with epoxy resin and is likely to produce a bloom of the epoxy resin. This makes the resin layer surface tacky, which induces the disadvantage of reduction in workability and handleability.
The steel plate reinforcing sheet whose resin layer consists of rubber-based material only has good workability but has low strength. In order to achieve satisfactory reinforcing effect, it is necessary to increase the thickness of the resin layer. However, such increase is likely to prevent the weight saving in the structure and cost reduction.